This invention relates to in-situ remediation of contaminated heterogeneous soils. In one aspect, this invention relates to a novel process combining electroosmosis and/or electromigration, hydraulic flow and in-situ treatment of contaminants in treating zones using biological, physicochemical, or electrochemical means. In a further aspect, this invention relates to a novel process for the in-situ remediation of soils contaminated with toxic organic compounds and/or toxic ionic contaminants such as metals and radionuclides.
Generally, degradation of toxic organic compounds to innocuous products such as CO.sub.2 and water can be accomplished either biologically or physicochemically provided the treatment is carried out in a well-controlled environment in which key operating parameters such as temperature, pressure, mixing, addition of the reactants or nutrients, etc., are optimized. Examples of these technologies include incineration and its variations, supercritical water oxidation, UV/H.sub.2 O.sub.2 /ozone/catalytic oxidation, reductive dehalogenation and biodegradation in an optimized bio-reactor. However, the cost associated with these technologies are high for the decontamination of soil, which must first be excavated and then processed into a form suitable for the particular reactor used. The reactor constitutes a major portion of the overall cost in these processes due to either the extreme conditions required with thermal approaches or the very long holding times required in biological approaches. To overcome these problems, destruction of the contaminants needs to be done in-situ to avoid the cost and complications associated with excavation and handling, and the process has to be energy efficient and mild to minimize capital and operating costs.
Many in-situ technologies have been proposed and developed for remediating contaminated soil and ground water. Since most sub-surface soils are heterogeneous, i.e., consisting of various zones of low permeability, e.g., clay soil, silty soil or fractured bedrock, within regions of high permeability, e.g., sandy soil or vice versa, such technologies are generally not very effective.
Hydraulic or pressure-driven flow, e.g., pumping or soil flushing, causes preferential flow in areas of high permeability. Slow contaminant diffusion from the low permeability zones into the preferential flow paths results in steady, low-level release of the contaminant and unsatisfactorily long clean up times. This is a major problem with conventional Pump and Treat technology which is the primary method utilized for remediating ground water contamination. Pump and Treat, where water is pumped from the contaminated aquifers, treated and then discharged, is rather ineffective with clean up times projected to be much longer than originally estimated. In cases of an immobile zone containing substantial quantities of absorbed contaminants or if non-aqueous phase liquids are present, the clean up times have been projected to be hundreds of years.
Due to the limitations of Pump and Treat, several enhancements to Pump and Treat have been developed and evaluated. These include reinjection of treated ground water, pulsing and in-situ bioremediation. However, these enhancement techniques have not demonstrated significant improvements to providing permanent solutions or reducing cost. Reinjection of treated ground water has been found to reduce cleanup times by up to 30% but without any reduction of cost. Pulsing of the Pump and Treat system has application where diffusion controls the release of contaminants but studies have found that cleanup times were longer even though cost may be lower because less water is treated. In-situ bioremediation will also not increase the cleanup rates of Pump and Treat systems where contamination release is diffusion controlled because cleanup time is still controlled by the diffusion from the immobile zone. In addition, little has been accomplished in enhancing cleanup time and achieving remediation goals if sufficient amounts of contamination are present in low permeability zones.
Various techniques have been suggested for application in processes for the in-situ remediation of low permeability contaminated soils. An example of such a technique is electroosmosis. However, electroosmosis as currently practiced suffers from limitations which make it commercially impractical.
Electrokinetics, specifically electroosmosis, has been suggested for use in in-situ remediation of soils contaminated with non-ionic, soluble organic compounds. Electroosmosis involves applying an electrical potential between two electrodes immersed in soil to cause water in the soil matrix to move from the anode to the cathode when soils are negatively charged, such as is the case with clay soils. When the soil is positively charged, however, the direction of the flow would be from the cathode to the anode. The technique has been used since the 1930's for removing water from clays, silts and fine sands. The major advantage for electroosmosis as an in-situ remediation method for difficult media, e.g., clay and silty sand is its inherent ability to get water to flow uniformly through clay and silty sand at 100 to 1,000 times faster than attainable by hydraulic means, and with very low energy usage. Electroosmosis has two major limitations as currently practiced that makes it impractical for actual field remediation. First, the liquid flow induced by electroosmosis is extremely slow, i.e., about 1 inch per day for clay soils, which could result in a cumbersome and very long-term operation in large scale operations. Second, several laboratory studies, (see Bruell, C. J. et al., "Electroosmotic Removal of Gasoline Hydrocarbons and TCE from Clay", J. Environ. Eng., Vol. 118, No. 1, pp. 68-83, January/February 1992 and Segall, B. A. et al., "Electroosmotic Contaminate-Removal Processes", J. Environ. Eng., Vol. 118, No. 1, pp. 84-100, January/February 1992) have indicated that part of the soil bed became dry after approximately 1 month under the electroosmotic effect, resulting in reduced flow and the eventual stoppage of the process. Another laboratory study (see Shapiro, A. P. et al., "Removal of Contaminants From Saturated Clay by Electroosmosis", Environ. Sci. Technol., Vol. 27, No. 2, pp. 283-91, 1993) has indicated that the acid generated at the anode moves through the soil bed in the direction of the cathode and results in reduced electroosmotic flow and eventual stoppage of the process.
In addition, electroosmosis generally is ineffective for soils of relatively high permeability, e.g., relatively loosely packed sandy soils. Typically for a voltage gradient of 1 V/cm, electroosmotic permeability is in the range of 10.sup.-5 to 10.sup.-4 cm/sec. In comparison, hydraulic permeabilities of sandy soils are normally &gt;10.sup.-3 cm/sec. Thus for heterogeneous soils, once the liquid exits the low permeability zone it is no longer under the effective control of electroosmotic force and hydraulic force and/or gravity will dominate the flow direction of the liquid. This is the major reason that electroosmosis has been viewed as limited to applications for treating low permeability soils having a hydraulic permeability in the range 10.sup.-8 to 10.sup.-4 cm/sec.
Several techniques have been suggested for application in processes for the remediation of soils contaminated with ionic contaminants such as heavy metals and radionuclides. Ex-situ techniques, e.g. separation, involves removing the soil containing ionic contaminants and treating the soil ex-situ to remove contaminants. Examples of separation techniques include soil washing and extraction. However, ex-situ methods are not commercially acceptable due to economic considerations resulting from the required excavation and treatment of the contaminated soil. In situ methods include electromigration and immobilization.
Electrokinetics, specifically electromigration, involves applying an electrical potential between two electrodes immersed in soil to cause solute, e.g. ions of metals, to migrate through a solution along the imposed voltage gradient, i.e. electromigratory movement. The charged species of metals in the soil migrate toward the oppositely charged electrodes and are collected at the electrodes. Electromigration has several limitations as currently practiced that make it impractical for actual field remediation. First, pH of the solution near the cathode tends to be very alkaline due to water electrolysis at the electrode and this causes most metals to precipitate in the soil making it difficult to remove the contaminants as well as blocking the flow of water through the contaminated soil region. Second, electrokinetics is inherently not a very stable process due to build-up of concentration, pH and osmotic gradients in the soil between the electrodes which adversely affect the process. In addition, the soil itself will also be altered over time, e.g. the soil will suffer from drying and cracking.
Immobilization encapsulates the contaminant in a solid matrix. Traditional immobilization options for heavy metal contaminated soil are solidification/stabilization (S/S) and vitrification. Traditional S/S methods produce monolithic blocks of waste with high structural integrity. However, the presence of hydrocarbons interfere with the S/S matrix and can increase the leachability of heavy metals when metals partition into the organic phase. Vitrification involves heating the contaminated soil to form chemically inert materials, e.g. glass. In vitrification, large electrodes are inserted into soil that contains significant levels of silicates. An electrical current is applied and the heat generated melts the soil and contaminants gradually working downward through the soil. The contaminants in the fused soil are not likely to leach. However, neither immobilization or vitrification is an economical commercial process.
Soil contaminated with toxic organic compounds and heavy metals and/or radionuclides present additional problems since remedial schemes for one type of contamination are often inappropriate for the other. For example, traditional remediation techniques for organic compounds such as bioremediation, incineration and thermal desorption are generally ineffective on heavy metals. In addition, the presence of most heavy metals can have toxic effects on microorganisms utilized to degrade organic contaminants. Treatment of mixed waste contamination typically requires a combination of various methods resulting in higher costs which are unacceptable.
An in-situ remediation process for use in heterogeneous soil regions which is commercially practical and economical, and solves the above-problems with the currently known technologies would be highly desirable. It has now been found that a combination of electrokinetics, pressure-driven or hydraulic flow and in-situ contaminant degradation in treating zones using biological, physicochemical or electrochemical means solves the above-described problems.